I have the following shape. It was created by creating four
ends and then creating a surface using either surface loft or
boundary surface between each of the four ends. Each surface is
colored a different color so you can tell what is going on.

I need to "fill in" the top dimple in between the four surfaces so
that everything between there is smooth - almost as if this were a
real part and you poured water or melted metal into the dimple. The
surface would be tangent with all four of the surfaces at the top
of each surface. It may or may not be a plane.

I asked this same question in much more detail in the
following link. You can visit it for a detailed answer. But
basically in your situation, use the "filled surface" tool, and set
all the edges to "curvature".